1. Field of the Invention
Embodiments of the present invention relate to semiconductor control devices for control of a switching element connected to a flyback transformer of a switching regulator.
2. Description of the Related Art
A flyback type switching regulator has been proposed having a circuit construction as shown in FIG. 5, in which the switching element connected to the primary winding of the flyback transformer is controlled depending on whether the load connected to the secondary winding of the flyback transformer is in a light load condition or in a heavy load condition.
In the conventional example of FIG. 5, an AC voltage delivered by an AC power source 100 is converted to a DC voltage by a rectifying circuit 101, which in turn is supplied to one terminal of a primary winding L1 of a flyback transformer 102. The other terminal of the primary winding L1 is connected to the ground through a series-connected circuit consisting of a switching element 103, which is a field effect transistor, and a current sense resistance 104.
A load 105 is connected to the secondary winding L2 of the flyback transformer 102. The voltage supplied to this load is detected by a shunt regulator circuit 106 and a photo-coupler 107.
An output voltage with a waveform similar to that of the primary winding L1 of the flyback transformer 102 is obtained by an auxiliary winding L3, which is tertiary winding, with a polarity opposite to that of the primary winding L1.
A semiconductor control device 110 is provided for ON-OFF control of the switching element 103. The semiconductor control device 110 has: a power supply input terminals VH and VCC, a latch terminal LAT, a feedback signal input terminal FB, a current detection signal input terminal IS, an output terminal OUT, and a ground terminal GND. An output voltage from the auxiliary winding L3 is delivered to the power supply input terminal VCC; a current detection signal Vi, which is a voltage signal detected on a current sense resistance 104, is delivered to the current detection signal input terminal IS; and a feedback signal Vf given by a photo-coupler 107 is delivered to the feedback signal input terminal FB. The semiconductor control device 110 executes predetermined signal processing based on the delivered current detection signal Vi and the feedback signal Vf to generate a gate driving signal Vg for driving the switching element 103. The gate driving signal Vg is delivered to the gate terminal of the switching element 103.
More specific construction of the semiconductor control device 110 is shown in FIG. 6. This semiconductor control device 110 comprises an oscillator 111, a one-shot circuit 112, an RS flip-flop circuit 113, a comparator 115, and a gate driver 116. The oscillator 111 generates oscillation with a frequency varied based on the feedback signal Vf. The one-shot circuit 112 generates a one-shot pulse at the moment of ON reversal, which is a change from an L (low) level to an H (high) level, of the output of the oscillation output of the oscillator 111. The RS flip-flop circuit 113 receives the one-shot signal from the one-shot circuit 112 at a set terminal of the RS flip-flop circuit. The comparator 115 compares the current detection signal Vi and a voltage Vf′ that is the feedback signal Vf level-shifted by a level shift circuit 114, and delivers a high level signal resulting from the comparison to a reset terminal of the RS flip-flop circuit 113 when the current detection signal Vi reaches the voltage Vf′. The gate driver 116 drives the switching element 103 receiving a logical sum signal of the one-shot signal from the one-shot circuit 112 and the output from the RS flip-flop circuit 113.
The feedback signal Vf is a signal indicating a magnitude of the load. Heavy loading condition, in which a large current is running in the load 105, gives a large value of the feedback signal Vf; and light loading condition, in which a small current is running in the load 105, gives a small value of the feedback signal Vf. Utilizing this characteristic, the oscillator 111 is designed in such a manner, as shown in FIG. 7, that a heavy loading condition corresponding to a rather large feedback signal Vf greater than a predetermined voltage Vf2 gives a constant frequency of the maximum frequency fmax, and a light loading condition corresponding to a feedback signal Vf smaller than the predetermined voltage Vf2 gives a frequency f that decreases with decrease of the feedback signal Vf and eventually becomes a fixed frequency of the minimum frequency fmin at a Vf value of a predetermined voltage Vf1 or less, the voltage Vf1 being sufficiently smaller than the voltage Vf2.
Operation of this conventional example is described in the following with reference to FIG. 8.
Referring to FIG. 8, the waveform (a) shows a gate driving signal Vg, the waveform (b) shows an electric potential Vd at the connection point between the primary winding L1 of the flyback transformer 102 and the switching element 103, and the waveform (c) shows the current detection voltage Vi. The oscillator 111 of the semiconductor control device 110 delivers an oscillation signal at a frequency f corresponding to the feedback signal Vf as shown in FIG. 7 to the one-shot circuit 112, which outputs a one-shot signal Ss. As a result, the RS flip-flop circuit 113 is set and the gate driving signal Vg at a high level as shown by the waveform (a) in FIG. 8 is delivered by the gate driver 116 to the switching element 103, to control the switching element 103 to the ON state.
DC current rectified by the rectification circuit 101 flows through the primary winding L1 of the flyback transformer 102, the switching element 103, and the current sense resistance 104, to the ground.
The current flowing in the switching element 103 is detected by the current sense resistance 104 as a voltage signal of current detection voltage Vi. The current detection voltage Vi is given to the current detection signal input terminal IS of the semiconductor control device 110. The current detection signal Vi varies as the waveform (c) of FIG. 8, for example. The voltage delivered to the load 105 that is connected to the secondary winding L2 of the flyback transformer 102 is detected by the shunt regulator circuit 106 and delivered as a feedback signal Vf to the feedback signal input terminal FB of the semiconductor control device 110 through a photo-coupler 107.
The comparator 115 executes comparison between the voltage Vf′ and the current detection voltage Vi. At the moment the current detection voltage Vi reaches the voltage Vf′, a signal at a high level resulted from the comparison is delivered to the reset terminal of the RS flip-flop circuit 113, to reset the RS flip-flop circuit 113. As a result, the gate driving signal Vg delivered by the gate driver 116 becomes a low level as shown in the waveform (a) of FIG. 8, turning the switching element 103 OFF.
The one-shot signal Ss delivered by the one-shot circuit 112 is used as a blanking signal for inhibiting malfunction due to turn ON noise, as shown in FIG. 9. The one-shot signal Ss is given as a gate driving signal through the OR gate 117, to provide a blanking time with the time duration of the one-shot signal Ss. If a turn ON noise is generated as shown by the curve (c) of FIG. 9, malfunction of turn OFF of the semiconductor element 103 is inhibited.
In the heavy loading condition of the load 105 in the side of the secondary winding L2 of the flyback transformer 102, the oscillation frequency of the oscillator 111 in the semiconductor control device 110 is fixed at the maximum frequency fmax as shown in FIG. 7. The switching element 103 repeats turning ON and turning OFF at the maximum frequency fmax. Here, the frequency characteristics of the flyback transformer 102 is designed so that the switching element 103 operates in a continuous current mode, in which the switching element 103 makes the next turn ON operation during the flyback transformer 102 is transmitting an electric power to the secondary side.
On the other hand, in the light loading condition, the oscillation frequency of the oscillator 111 decreases as shown in FIG. 7. In the loading below a certain degree, the operation becomes a discontinuous current mode, in which the switching element 103 makes the next turning ON operation after the flyback transformer 102 finishes transmitting an electric power to the secondary side. In the discontinuous current mode operation, after completion of power transmission to the secondary side, LC resonance oscillation arises due to an inductance component of the primary winding L1 of the flyback transformer 102 and capacitance components of a parasitic capacitance of the switching element and a capacitance of a noise suppression capacitor (not shown in the figures). Consequently, the voltage Vd varies as shown in the waveform (b) in FIG. 8.
Because the turn ON timing of the switching element 103 is regulated by the oscillator 111 in the semiconductor control device 110, it is determined at random whether turning ON of the switching element 103 occurs at a moment of high voltage in the resonance oscillation of the drain voltage Vd, which is referred to as ‘top turning ON’, or at a moment of low voltage in the resonance oscillation, which is referred to as ‘bottom turning ON’.
Owing to the decreased switching frequency, the switching loss in the switching element 103 is smaller than the magnitude in the heavy loading condition. However, the switching loss is rather large in the top turning ON condition in which turning ON occurs at a moment of high voltage in the resonance oscillation due to discharge of the capacitance component in the LC resonance circuit, as compared with the bottom turning ON condition in which turning ON occurs at a moment of low voltage in the resonance oscillation.
In order to deal with this problem, it has been considered to provide a bottom detection circuit outside the semiconductor control device 110 for detecting a bottom condition during switching of the switching element 103. To provide such a bottom detection circuit, a dedicated input terminal is needed for delivering the bottom detection signal from the bottom detection circuit to the semiconductor control device 110. Thus, problems arise due to the increased number of pins in the semiconductor control device 110 and increased cost of the semiconductor control device 110.
To cope with the problems, Non-patent Document 1 discloses a switching regulator that performs detection of a bottom condition in the drain voltage Vd of a MOSFET connected to the primary winding of the flyback transformer. This detection of bottom condition is carried out by delivering a voltage given by an auxiliary winding L3 of the flyback transformer 102, the voltage having a similar waveform to the one in the primary winding, to an overcurrent protection input/bottom detection terminal, which corresponds to the current detection signal input terminal IS. In the LC resonance oscillation state in this switching regulator, a first control mode in the heavy loading condition and a second control mode in the light loading condition are interchanged. In the first control mode, the bottom turn ON of the MOSFET is conducted on detection of the bottom condition in the heavy loading condition. In the second control mode, the bottom turning ON of the MOSFET is conducted when the bottom condition is detected twice after once skipping the bottom condition in the light loading condition. Thus, the switching frequency is decreased in the light loading condition and the switching loss is reduced.
In the conventional example disclosed in “STR-X6700 Series Application Note (Rev. 1.3),” pages 12-14, delivered by Sanken Electric Co., Ltd. (Searched and found on Oct. 6, 2010 on the Internet) (referred to herein as “Non-patent Document 1”), the control mode is interchanged between the first control mode in which skip of bottom condition is not conducted in the heavy loading condition and the second control mode in which skip of bottom condition is conducted once in the light loading condition.
The light loading there means a short period of time for storing energy in the flyback transformer and a short period of time for transmitting the energy from the primary side to the secondary side of the flyback transformer. A switching period is a sum of the energy storage and transmission time and one or two periods of the free oscillation. Therefore, a lower loading condition generally results in a higher switching frequency, increasing energy loss.
In the control device of Non-patent Document 1, when the loading becomes light, the number of skip of the bottom condition is increased from ‘0’ to ‘1’, reducing the switching frequency at that moment. After that time, however, the switching frequency increases with decrease of loading, this is one of the problems in the control device of Non-patent Document 1. In the heavy loading condition, since the resonant oscillation is presumed in this conventional device, the continuous current mode operation cannot be performed.